The present invention relates to an accelerometer comprising a supporting base and at least one electromechanical transducer constituted of at least one sensitive element, each of said at least one sensitive element comprising at least one piezoelectric element, the whole being arranged between said supporting base and a seismic mass.
It is known to use piezoelectric accelerometers for measuring the vibrations of rotating machines such as gas turbines or jet engines. Among the known basic principles used for the design of accelerometers, there are two that are the most frequently used, namely, the shear mode design and the compression mode design. The compression mode designs can be split in two subgroups. A first subgroup using a pure compression of the piezoelectric material (i.e. d33, d11 or d22 mode) whereas a second subgroup uses the transverse compression of the piezoelectric material (i.e. d31, d21, d32, d23 , d13, d12 mode). These two basic designs use at least one seismic mass which, under the effect of the acceleration generated by the vibrations, acts upon one or more piezoelectric elements.
In the shear mode accelerometer design, a deformation of the piezoelectric elements produced by a shear force is used for producing an electric charge, whereas in the compression mode accelerometer design, a longitudinal compression of the piezoelectric elements in the direction of the sensitive axis produces an electric charge.
Each one of these two basic accelerometer designs has advantages and limitations for the design engineer, but also in the practical implementation. Shear mode accelerometers can be very small devices for a given sensitivity, but have a lower resonance frequency. Compression mode accelerometers are known to be suitable for applications which require a high resonance frequency. In view of the piezoelectric properties of certain materials, design engineers prefer compression mode accelerometers for high temperature applications.
Serious problems may arise when piezoelectric accelerometers are excited by high vibration levels in the transverse axis, i.e. in a direction normal to the sensitive axis. This problem is encountered more specifically when the transversal vibration excitation frequency coincides with the transversal resonance of the transducer whereby the movement is amplified. Under these conditions, the bending stress on the piezoelectric elements can produce a significant noise in the output signal to such an extent that this output signal is substantially corrupted. In the past, this phenomenon has very often not been correctly understood, and the results could be interpreted as a pyroelectric effect or an effect of a sensitivity to strain of the supporting base (base strain).
The main aim of the invention is to overcome the above-indicated problems and to provide an accelerometer having a structure that eliminates or at least substantially reduces the influence of transversal accelerations on the accuracy of the measurement results.
Another important and essential aim of the invention is in particular to provide a compression mode accelerometer having a structure that eliminates or at least substantially reduces the influence of transversal accelerations on the accuracy of vibration signal provided by the accelerometer.
According to the invention the above aims are attained with an accelerometer comprising:
a supporting base,
at least one electromechanical transducer each of which is fastened on said supporting base, and each of which includes at least one seismic mass, and at least one sensitive element, each such sensitive element comprising at least one piezoelectric element,
a structural element which is rigidly connected to said supporting base, and
at least one laterally stabilizing element which is in mechanical contact with said at least one electromechanical transducer and with said structural element.
In a preferred embodiment of an accelerometer according to the invention, the laterally stabilizing element is substantially more rigid in radial than in axial direction. The above mentioned structural element may constitute a part of an accelerometer housing which housing is rigidly fastened to the supporting base.